Method of hypochlorite production and related sea water electrolyzer with anti scale implemen

ABSTRACT

A significant decrement of the rate of accumulation of scale on electrode and other inner surfaces of a sea water electrolyzer of an electro-chlorination plant, is achieved by intermittently ejecting pressurized sea water through arrays of nozzles that produce energetic (high speed) jets adapted to encroach deep inside the interelectrodic gap spaces of an elongated pack of planar parallel interleaved electrodes. The kinetic energy of the jets is gradually absorbed by the process liquid flowing through the gap spaces from the inlet end to the outlet end of the tubular casing of the electrolyzer containing the elongated electrode pack. The input of kinetic energy in a transverse or inclined direction of encroachment of the jets in the liquid flowing in the gap spaces disrupt the laminar flow causing an energetic turbulence in the interelectrodic body of liquid which effectively hinder a stable accumulation of scale, a good portion of which is swept by the liquid stream out of the electrolyzer into drains in the lower part of electrolyzer.

TECHNICAL FIELD

The present disclosure relates in general to electrolytic hypochloritegeneration in diluted impure saline solutions like sea water and inparticular to a method and implement for contrasting build up of scaleover electrode and other surfaces inside the electrolyzer.

STATE OF THE ART

The electrolysis of sea water is commonly used for the direct productionof hypochlorite to prevent bio-fouling and scaling of cooling systems,water ballasts, and alike.

In the context of this disclosure, though generically referring to seawater, it must be intended that the description of the problems and ofthe solution found apply, mutatis mutandis, also to equivalent dilutedsaline solutions containing sodium chloride besides other salts andimpurities of organic and/or inorganic type, like brackish water bodiesof natural origin or of water bodies in densely populated urban areas,etc.

The process, which is much safer and more economical than the additionof gaseous chlorine into the sea water cooling circuit, suffers ofproblems connected to impurities and dissolved substances present in thesea water. It is known that sea water contains, in addition to sodiumchloride, which is the starting material for the production ofhypochlorite, many other ions that interfere with the process. Thefundamental reactions occurring when a direct current flows through thesea water in the electrolyzer (often referred to also with thefunctional name of electro-chlorinator) are the following:

at the anode: generation of chlorine

2Cl⁻→Cl₂+2e⁻  1)

at the cathode: generation of hydrogen and formation of hydroxide ions

2H₂O+2e⁻→2OH⁻+H₂   2)

in the gap between the electrodes: interaction between chlorine andhydroxide ions that generates hypochlorite

Cl₂+2OH⁻→H₂O+ClO⁻+Cl⁻  3)

The hydrogen bubbles that forms inside the electrolyzer and that aretransported in the stream of water eventually surface in a stockingvessel of the hypochlorite containing solution to be diluted in an airstream that constantly vent it to atmosphere.

Hypochlorite has a specific oxidizing and sterilizing effect andregenerates the original chloride ion when contacting organic substancesor by effect of light, heat or of readily oxidizable ions, thus leavingno noxious residues in the sea water after the sterilization process.

Unfortunately, both at the anode, at the cathode and in the electrolytesolution (sea water) there occur numerous undesired chemical reactions,an exhaustive review of which may be read from U.S. Pat. No. 4,488,945,though it focus almost exclusively on electrochemical and chemicaltransformations originating from parasitic anodic reactions and onconsequent deactivation mechanisms of the metallic anodes. The pertinentdescription is intended herein incorporated by express reference.

Many other ions are generally or occasionally present in coastal waters.Calcium, magnesium, iodine, bromine, sulfur and many heavy metals ionsmay be found. In particular, calcium and magnesium are present inrelatively large amounts in sea water and cause a severe scaling of thesurface of the negatively biased electrodes and other surfaces. Thesescales are generally porous and do not interfere with the normalfunctioning of the cell, but their rapid growth makes them more and morecompact and cause an increase of the operating cell voltage andconsequent waste of electric power.

A commonly preferred architecture of electrolyzer for anelectro-chlorination plant is depicted in FIG. 1. A tubular casingcontains a pack of spacingly interleaved planar anodes and cathodesdefining interelectrodic gap spaces there between, practically composinga multiplicity of electrolytic cells, electrically parallelized (4 cellsin parallel) in a tranverse direction and serialized (5 cells in series)in the longitudinal direction of the tubular casing of the electrolyzer,connected to a DC source through a positive connector and a negativeconnector at the opposite ends of the tubular casing.

In particular, the growth of scaling in the interelectrodic gapincreasingly hinders a correct distribution of the flow of sea waterbetween the flat counter-opposed surfaces of the metal plate electrodesas far as clogging definitely the electrode pack of the electrolyzer.

A observable in the photograph of FIG. 2, an excessive scaling, if notpunctually controlled tends to progress in an exponential way and maybend the metal plate electrodes and eventually cause electrical sparksand even destructive short circuits, phenomena that may be fostered alsoby a concurrent process of galvanic deposition of metals.

Each acid wash phase usually takes from 3 to 5 hours with associatedloss of production besides the cost of the acid (commonly HCl), wasteliquor treatment/disposal and hazard management costs.

Even a gross pre-purification process of the sea water to be passedthrough the electrolyzer would be utterly uneconomical, given that theflown volume of sea water is generally between 400-1000 liters per kg ofchlorine generated at the anode.

Therefore, frequent acid washings of the electrode pack must be carriedout in order to remove accumulated scale and metal deposits. Dependingon the salinity of sea water, flow rate, current density and state ofthe electrodes, the interval between these scheduled maintenanceoperations may be as short as 10-25 days.

OBJECTIVE AND SUMMARY OF THE INVENTION

Objective of the applicant was to find a viable way to alleviate thefrequency and/or duration of the periodic down times of anelectro-chlorination plant, required for carrying out an effective acidwash of the interior of the electrolyzer after a period of operation andto reduce acid consumption and the power costs associated to suchunavoidable routine maintenance of the plant.

Field trials carried out by the applicant by varying the flow rate ofsea water through the tubular electrolyzer casing containing a pack ofspacingly interleaved planar anodes and cathodes defininginterelectrodic gap spaces there between, generally composing amultiplicity of electrolytic cells, electrically parallelized in atranverse direction and serialized in the longitudinal direction of thetubular casing of the electrolyzer, were not encouraging.

Even at flow rates largely exceeding the critical Reynold figure forensuring a turbulent flow through the electrolyzer, no significanthindrance of the scaling process of the electrodes was observed whilstthe increased pumping significantly penalized efficiency of thechlorination process. Even the deployment of flow diverting bafflesproved to have scarce or null effect.

This seemed to reinforce a largely predominant belief that turbulencewould probably worsen the problem of scaling because of an enhancedtransport of ions to the electrodes or, at best, be ineffective incontrasting scaling. However, the applicant did not give up on the ideathat a turbulent flow of sea water could be of aid in countering therapid build-up of scale in the interectrodic gap spaces.

Despite these early failures, the solution finally found by theapplicant has proven itself surprisingly effective.

Basically the idea has been to introduce into the stream of sea waterpumped through the tubular casing of the electrolyzer a plurality ofsecondary or ancillary streams of highly pressurized water in form ofhigh speed jets ejected from an array of nozzles, disposed at regularintervals along the whole length of at least one side of the elongatedpack assembly. The location of the many nozzles, their distance from theelectrode pack and the spacing intervals among each other are such toensure that high speed jets of liquid encroach (are directed toward)into the interelectrodic gap spaces, in a direction orthogonal orinclined in respect of the longitudinal flow direction of sea waterthere through; the sought effect being to disrupt a probably laminarflow of sea water within the interelectrodic gap spaces and induce anenergetic turbulence of the liquid therein.

A surprisingly remarkable decrement of the rate of accumulation of scaleon electrode and other inner surfaces of the electrolyzer wasimmediately observed, which appeared to confirm the applicant'ssuspicion that a honeycomb effect of the closely packed parallel plateelectrodes could have been responsible of vanifying his previousexperiments aiming to verify a possible benefit from turbulence.

The overpressure of the liquid to be injected should be sufficient toproduce energetic (high speed) jets adapted to encroach deep inside theinterelectrodic gap spaces. The kinetic energy of the jets is graduallyabsorbed by the liquid flowing through the gap spaces from the inlet endto the outlet end of the tubular casing of the electrolyzer containingthe elongated electrode pack. The input of kinetic energy in atransverse or inclined direction of encroachment of the jets in theliquid flowing in the gap spaces disrupt the laminar flow causing anenergetic turbulence in the interelectrodic body of liquid which appearsto effectively hinder a stable accumulation of scale, a good portion ofwhich is swept by the liquid stream out of the electrolyzer and/or fallsinto drain nozzles normally present in the lower part of electrolyzer,from where the scale may be discharged.

A periodic ejection of jets of pressurized liquid may be carried outduring normal operation of the electrolyzer or the electrolysis processmay be interrupted during the ejection phase. Preferably, interruptionof the DC electrical powering of the electrodes and of the forced flowof the process liquid through the electrolyzer is commanded when thedischarge valve of drain nozzles present along the bottom of theelectrolyzer need to be opened for discharging accumulated scale,displaced by the jets from the electrode surfaces and sunk to thebottom.

Numerous test runs have clearly demonstrated that the high pressureinjection of orthogonal/inclined jets does not need to be continuous andthat an intermittent injection is preferable. Generally, for anelectrolytic process conducted at a current density of 1,000 to 1,800A/m², the intermittent high pressure scale sweeping phase may have aduration of about 5 min. and be carried out every 15-30 minutes.However, duration and frequency of the scale sweeping phase may beadjusted by a control PLC depending on operating conditions and qualityof the processed saline water. The PLC is also programmable forimplementing scale sweeping phases without interruption of theelectrochemical process and scale sweeping phases with simultaneousinterruption of the electrochemical process and opening of the bottomdischarge drains, according to needs or pre-established time schedules.

Generally the pressure of the injected liquid may be comprised between 5and 10 N/m2.

The ratio between the cumulative flow rate of liquid ejection throughsaid plurality of nozzles and the flow rate of said forced flow ofprocess liquid through the electrolyzer may be comprised between 0.2 and0.6.

The cumulative flow rate of the plurality of secondary or ancillarystreams of highly pressurized water in form of high speed jetspractically amounts to a fraction of the flow rate of the main flow ofsea water pumped through the electrolyzer. This limits the powerrequirement for pressurizing the liquid to be injected for asurprisingly low cost/benefit ratio when considering the overalleconomical figures of the electro-chlorination process.

An evaluation test, carried out on two of a battery of four identicalelectrolyzers CHLOROPURE™-HPSC (a trademark of S:E:S:P:I:srl-Milan-Italy) of a commercial electro-chlorination plant gave thefollowing results:

-   -   the long established frequency of routinely performed acid wash        steps of the interior of the electrolyzers not equipped with the        novel auxiliary injection system of the present disclosure,        every 18 days of continuous operation was maintained unchanged;    -   the frequency of the routinely performed acid wash steps of the        interior of the electrolyzers equipped with the novel auxiliary        injection system of the present disclosure, after few        adjustments following initial check runs, could be set to 165        days of continuous operation, achieving almost a tenfold        reduction of the duty cycle of periodic acid wash.

The invention is defined in the annexed claims, the content of which isintended constituting integral part of the present description andherein incorporated by express reference.

A description of embodiments of this invention with reference to theannexed drawings will follow, with the sole aim of making clear to theskilled artisan manners in which the invention may be practiced, withoutany intended limitation of its practice to the exemplary embodimentsshown and described in detail.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a basic cross sectional view showing the structure of anelectrolyzer commonly used for generating sodium hypochlorite in astream of sea water or equivalent saline solution.

FIG. 2 is a photograph and an enlargement of it of the electrode pack ofa commercial sea water electrolyzer, practically destroyed by anunchecked growth of scale.

FIGS. 3A, 3B, 3C and 3D provide a basic illustration of an electrolyzerfor sea water electrolysis embodying the novel anti scaling implement ofthis invention.

FIGS. 4A and 4B are a view from atop and an enlarged partial view,respectively, of the pre-assembled electrode pack of the electrolyzer.

FIG. 5 is an exploded detail view of a pressurized water distributorwith uniformly spaced nozzle assembly according to an exemplaryembodiment.

FIG. 6 is a general functional diagram of an electro-chlorination plantusing the novel electrolyzer of this invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 3A, FIG. 3B, FIG. 3C and FIG. 3D are a longitudinal cross sectionalview and end views without and with sectional views of the electrodepack ,off and in coincidence with one array of spacers, respectively, ofan exemplary commercial sea water electrolyzer for electro-chlorinationplants, equipped with the anti-scaling implements of this invention.

The electrolyzer has a tubular casing 1, commonly a horizontalcylindrical shell of PVC with an outer cladding of GRP, closed by endflanges (not shown), adapted to contain a pre-assembled multi electrodepack (C1, . . . , A1), the functional arrangement of which alreadydepicted in FIG. 1, may be recognized in the view of the pack from atopof FIG. 4A and in the partial enlarged drawing of FIG. 4B, where theinsulating spacers between the plate electrodes are more visible. Thetubular casing 1 has a sea water inlet nozzle 2 and an outlet nozzle 3through which hypochlorite containing sea water and hydrogen bubblesdispersed in the liquid stream exit the electrolyzer. The bottom nozzles4 serve for flush discharging the acid solution with which the interiorof the electrolyzer if filled when carrying out the periodic acidwashings. An optional nozzle 5 at the top may be provided, closed with atransparent pane, in order to provide a visual inspection window of thestate of the scale accumulation.

Referring to the longitudinal side view visible in FIG. 3A and to theorthogonal view of FIG. 4A, in the illustrated embodiment, the electrodepack assembly defines a multicell electrical series formed by planartitanium plate electrodes, 1 mm thick, each having, over the anodicallypolarized portion (A1, A2, A3, . . . , A7) of the plate, a nonpassivating, conductive metal oxide coating with a low chlorine ionoxidation overvoltage and without any coating or with a low hydrogen iondischarge overvoltage metallic coating over the cathodically polarizedportion (C1, C2, C3, . . . , C7). A terminal cathodic head (C1) and aterminal anodic head (A1), each generally in form of a about 17 to 65teeth comb-like assembly of parallel cathode plates and anode plates,respectively, (refer also the basic functional scheme of FIG. 1) connectto a DC power source via rods (electrical connection leads) passingthrough closing end flanges of the tubular casing 1.

The novel implements of this invention, according to the exemplaryembodiment illustrated in FIG. 5, consists of two additional nozzles 6 tand 6 b, respectively at the top and at the bottom of the casing 1,accommodating a flanged inlet pipe 7 t and 7 b, respectively, a threadedend of which tightens in a respective “T” coupling between the twoportions 10 t (and the two portions 10 b) of a top and a bottomdistributor pipe of high pressure water, closed at both ends.

Of course, alternative configurations of longitudinal distributorsextending for the whole length of the electrode plate pack, above andunderneath the top and the bottom edges of the vertically held parallelmetal plates of the electrode pack are possible, for example, thedistributor pipe may be in a single piece, one end of which passingthrough one of the end closing flanges of the tubular casing 1 forallowing a direct external hydraulic connection.

Though a single centrally positioned distributor may be sufficient, incase of larger electrode packs, two or more parallel distributors may besimilarly deployed, in the top part and in the bottom part of theinterior space of the tubular casing. In any case, each distributor pipehas a plurality of equally spaced and similarly oriented nozzles 9 t and9 b, respectively, adapted to release highly energetic jets (high exitvelocity of the pressurized water) directed toward the spaced edges ofthe parallel electrode plates and the related interelectrodic gap spacesalong which flows the stream of sea water pumped through theelectrolyzer.

In the illustrated embodiment, the electrode pack assembly had thefollowing dimensions: length 274 cm, width 24,8 cm, height 16,6 cm; thedistributors were of ¾ A″ PVC pipe and had 17 threaded PVC nozzlescoupled to the pipe distributor at 150 mm intervals. The nozzles wereset at a distance of 60 mm from the plane of the spaced edges of theparallel plate electrodes.

The preferred main construction material for piping, flanges internaland external supports, hydraulic seal fixtures and other fixtures isgenerally PVC, though other plastics may be alternatively used.

FIG. 6 is a simplified general functional diagram of anelectro-chlorination plant embodying the novel implements of thisinvention. The scheme and the legend of the symbolic representation ofthe functional element make the illustration perfectly legible by theskilled person and no specific description is deemed necessary for thefullest comprehension of the claimed invention.

Various modifications of the process and apparatus of the invention maybe made without departing from the spirit or scope thereof and it is tobe understood that the invention is intended to be limited only asdefined in the annexed claims.

1. A process of electrolysis of sea water or equivalent saline water toproduce hypochlorite therein in at least an electrolyzer having aplurality of spacingly interleaved vertically held planar anodes andcathodes defining interelectrodic gap spaces there between, throughwhich said sea water or equivalent saline water is forced to flow at agiven flow rate, constituting a multi electrode pack assembly containedinside a tubular casing of the electrolyzer, comprising the step ofintermittently ejecting, through a plurality of nozzles disposed atregular intervals along the whole length of at least the top side ofsaid pack assembly, pressurized water, sea water or equivalent salinewater, jets of which are directed towards said interelectrodic gapspaces, orthogonally and/or at an inclination to the streaming directionof sea water or equivalent saline water forced to flow therein.
 2. Theprocess of claim 1, wherein the orthogonally directed jets disruptlaminar liquid flow in said interelectrodic gap spaces.
 3. The processof claim 1, wherein the liquid being ejected is at a pressure comprisedbetween 5 and 10 N/m².
 4. The process of claim 1, wherein the ratiobetween the cumulative flow rate of ejection through said plurality ofnozzles and the flow rate of said forced flow is comprised between 0.2and 0.6.
 5. The process of claim 1, wherein said jets directed into saidinterelectrodic gap spaces are simultaneously ejected from atop and fromunderneath said pack assembly.
 6. An electrolyzer system forelectrolyzing sea water or equivalent saline water to producehypochlorite, the electrolyzer having a plurality of vertically held,spacingly interleaved planar anodes and cathodes defininginterelectrodic gap spaces there between and constituting a multielectrode pack assembly contained inside a tubular casing of theelectrolyzer through which said sea water or equivalent saline water isforced to flow at a given flow rate, further comprising: at least alinear array of nozzles at regular intervals along a distributor pipeextending inside said casing for the whole length of at least the topside of said multi electrode pack assembly; an intermittent externalsource of pressurized water, sea water or equivalent saline water,connected to said internal distributor pipe for ejecting from saidnozzles jets directed toward said interelectrodic gap spaces,orthogonally to the streaming direction of sea water or equivalentsaline water forced to flow therein.
 7. The electrolyzer of claim 6,further comprising at least a second linear array of nozzles at regularintervals along a second distributor pipe extending inside said casingfor the whole length of the bottom side of said multi electrode packassembly for simultaneously ejecting jets from atop and from underneathsaid pack assembly.